1. Field of the Invention
The present invention relates to a magnifying optical system for endoscope, and more specifically an objective lens system (optical system) for endoscope which has a vari-focal function, thereby permitting observation of a magnified image.
2. Description of the Prior Art
In recent years, an increasing demand is posed in a medical field for an optical system which permits observation of a magnified image for precise diagnosis of a location which has changed to a morbid state.
Optical systems which are disclosed by official patent publications mentioned below are known as conventional examples of optical system which permits observation of such a magnified image, and is of a type which consists of three positive, negative and positive lens units, and performs a magnification change and focusing by moving the second negative lens unit:                Japanese Patent Kokoku Publication No. Sho-61-44283        Japanese Patent Kokai Application No. Hei 6-317744        Japanese Patent Kokai Application No. Hei 11-316339        
Furthermore, a conventional example which is disclosed by the following official patent publication is known as an optical system which is composed of three negative, positive and negative lens units, and configured to perform a magnification change and focusing by moving the second positive lens unit:                Japanese Patent Kokai Publication No. 2000-267002        
In order to enhance diagnostic precision, on the other hand, demands are posed for endoscopes which form image of higher qualities and it has been started to adopt image pickup devices on which picture elements are arranged in a number larger than conventional. It is therefore necessary to configure an image pickup optical system so as to have high performance which copes with such image pickup devices on which the picture element are arranged at large numbers.
In case of observation with an endoscope, for example, it is demanded to obtain an image having an image quality which is not lowered from a far point to a near point in an observation depth of an image pickup optical system, and it is therefore necessary for meeting this demand to use an image pickup optical system which has nearly pan-focus performance which is obtained by enlarging an large F number, or reducing an aperture of an aperture stop.
On the other hand, it is necessary to configure an image pickup optical system so as to satisfy the following Rayleigh limit condition to prevent an image quality from being lowered due to a diffraction phenomenon which is allowed to occur when an F number is enlarged, or an aperture of an aperture stop is reduced:Fno<1.64P/λ                wherein a reference symbol Fno represents an F number of an image pickup optical system, a reference symbol P designates a pitch of picture elements arranged on the image pickup device and a reference symbol λ denotes a wavelength.        
It will be understood from the above-mentioned formula that so large an F number cannot be obtained as picture elements are arranged at a smaller pitch P or, or in a larger number of an image pickup device. Accordingly, arrangement of picture elements in a larger number of an image pickup device may result in an insufficient observation depth of an image pickup optical system, thereby making it impossible to obtain a desired image quality.
In a case where a magnifying endoscope is set in a condition for observing a magnified image of an object at a short distance (at a tele position) in particular, the short distance between an objective lens system and the object makes it necessary to reserve a sufficient image quality on a side of a near point of an observation depth and an aperture of an aperture stop is usually reduced to a diffraction limit.
In this case, a conventional magnifying endoscope poses no problem in practical use since the endoscope provides a sufficient observation depth even when an F number is changed by moving a lens system from a condition for observing a magnified image of an object at a short distance (tele position) to a usual observing condition (wide position).
In the optical system disclosed by Japanese Patent Kokoku Publication No. Sho 61-44283, Japanese Patent Kokai Publication No. Hei 6-317744 and Japanese Patent Kokai Publication No. Hei 11-316339 out of the above-mentioned conventional examples, movable lenses are disposed after aperture stops and exit pupils are moved from the image side to the object side for observing magnified images. For this reason, F numbers in usual observing conditions are smaller than those in conditions for observing magnified images.
When the conventional optical system disclosed by these patent publications are used in combination with image pickup device on which picture elements are arranged in large numbers, the optical systems will have too small F numbers in the usual observing conditions even if the F numbers are reduced to the diffraction limit, thereby having short depths and posing problems in practical use.
Furthermore, Japanese Patent Kokai Publication No. 2000-267002 discloses a magnifying optical system for endoscope of a type having an F number which is not different between a usual observing condition and a condition for observing a magnified image.
The magnifying optical system for endoscope disclosed by this patent consists of three negative, positive and negative lens units, and performs a magnification change and focusing by moving the second lens unit.
However, since this optical system uses only one positive lens unit, the movable second lens unit tends to have strong power.
Accordingly, this optical system allows aberrations to be changed remarkably between a usual observing condition and a condition for observing a magnified image, and cannot meet a demand for a high image quality. This optical system allows a remarkable change of chromatic aberration in particular which is a cause of image color oozing.
Furthermore, Japanese Patent Kokai Publication No. 2000-67002 discloses an optical system consisting of three positive, negative and positive lens units, out of which the second lens unit is movable. However, an optical system of this type allows an F number to be changed remarkably between a usual observing condition and a condition for observing a magnified image, thereby posing a problem which is similar to those posed by the optical systems disclosed by Japanese Patent Kokoku Publication No. Sho 61-44283, Japanese Patent Kokai Publication No. Hei 6-317744 and Japanese Patent Kokai Publication No. Hei 11-316339.
Furthermore, the above-mentioned Japanese Patent Kokoku Publication No. Sho 61-44283 discloses an objective lens system for endoscope having a high magnification, which is problematic for practical use since the optical system has a narrow visual field in a usual observing condition, thereby making it difficult to perform such works as screening interiors of living bodies for finding morbid portions, treatment of the morbid portions and the like.